1. Field of the Invention
The present invention relates to an alternator to be mounted on an automotive vehicle.
2. Description of Related Art
It is important, in an automotive alternator, to prevent abnormal abrasion of slip-rings caused by water, oil or foreign particles entering onto a sliding surface of the slip-rings. It is also important to exhaust brush powders to suppress heat generated by abrasion. For these purposes, JP-A-2000-139058 proposes an alternator having a brush device, in which a separating wall formed in a slip-ring cover is inserted into a depressed portion formed in a brush holder. A ventilation passage is surely secured in this brush device because the ventilation passage is formed by a resin material, of which the brush holder and the slip-ring cover are made, without using deformable sealing rubber. JP-A-2002-359951 proposes another alternator that includes a slip-ring cover having a ventilation window in a peripheral wall surrounding the slip-rings. In this brush device, no ventilation opening is formed at an axial end of a brush holder.
In the alternator proposed in JP-A-2000-139058, however, an axial length of the brush device becomes long because the depression has to be formed in the brush holder at its axial end. In the alternator shown in JP-2002-359951, it is difficult to prevent water from entering into the brush device because the ventilation opening is formed on the peripheral wall of the slip-ring cover. If brushes are positioned downward of the slip-rings, water easily enters into a space containing the brushes. Further, there has been a problem that water sneaks-in due to interfacial tension from a portion where the brush holder contacts the slip-ring cover, even if water does not directly enters from the peripheral opening.